1. Field of the Invention
This invention relates to a video projector, and particularly relates to a color video projector constructed in such that color component images of R (red), G (green) and B (blue) of an original image respectively displayed on a plurality of CRTs provided to respective colors of R, G and B are focused onto a plurality of spatial light modulators for amplifying intensities of the color component images, and a single 3-color image of composite light of R, G and B obtained through the spatial light modulators and a cross-type 3-color composition member, is projected onto a screen as a magnified color composite image by a projecting lens system having only one optical axis.
2. Description of the Related Arts
Recently, diversity and high quality of image information have been advanced, so that there are increased such high quality image data as typically presented in HDTV (high-definition TV) broadcasting system and SVGA (Super Video Graphic Array) of a computer graphic system. In this connection, there are proposed many kinds of video projectors for displaying the high quality image data with a large magnification.
In these video projectors, there is one in which an image displayed on a CRT (cathode ray tube) is directly projected onto a screen by being optically magnified, and another one in which a small liquid crystal panel is driven as a light valve and the image displayed on the light valve is projected onto a screen by being optically magnified.
In these methods, it requires a high brightness for the image to be displayed on the CRT or on the liquid crystal panel to project an adequately bright image on the screen.
Specifically, in the case of employing the CRT, it may be possible to obtain a bright image by increasing the electron beam output of the CRT, however, it is impossible to obtain a high resolution image at the same time due to the inherent limitation of the CRT.
On the other hand, in the case of employing the liquid crystal panel, it is well known that the brightness of the image displayed on the panel depends on an amount of a light passing through the liquid crystal panel caused by light emitted from a light source placed in backside thereof. Therefore, when an area of a window or a picture element is made larger so as to increase the amount of the light passing through the liquid crystal panel, the numbers of the picture elements of the liquid crystal panel are obliged to be more decreased, this degrades the resolution of the image thereon. Thus, it is impossible to obtain the image having both the high resolution and the high brightness in using the above methods.
Therefore, there is proposed a video projector capable of projecting an image having both the high resolution and the high brightness on the screen in such a manner that the image displayed on the CRT or the liquid crystal panel is focused onto a spatial light modulator (trade name, ILA: Image Light Amplifier) having a function of a light amplification as shown in "Electronics Life" September 1993 (page 117-121) issue.
FIG. 1 is a schematic sectional side view for explaining the principle of the video projector in the prior art;
FIG. 2 is a sectional side view showing a spatial light modulator used in FIG. 1; and
FIG. 3 is a perspective view showing a color video projector employing a plurality of CRTs and a plurality of spatial light modulators each provided for R, G and B.
Referring to FIG. 1, the projector 1 in the prior art comprises a CRT 2, a writing lens 3, a spatial light modulator 4, a reading light source 5, a polarization beam splitter 6, a projection lens 7, and a screen 8, wherein the projector 1 is constructed in such a manner that an image displayed on the CRT 2 is focused onto the spatial light modulator 4 by the writing lens 3, and the image is projected in a large magnification onto the screen 8 by the projection lens 7 through the spatial light modulator 4 having a function as an image light amplifier. A liquid crystal panel (not shown) may be used instead of the CRT 2.
Before giving an operational explanation of the video projector 1, a description is given of the construction and operation of the spatial light modulator 4 in reference with FIG. 2.
As mentioned in the foregoing, the spatial light modulator 4 has a function as an image light amplifier for amplifying a feeble writing light (image picture), of which the amplification is about millionfold.
The spatial light modulator 4 is composed of a several thin films interposed between a pair of glass substrate 4a, 4b, without undergoing a minute dividing process for the picture elements.
Specifically, the spatial light modulator 4 comprises a glass substrate 4a, a transparent electrode 4c, a photo-conductive layer 4e, a light shielding layer 4f, dielectric mirror 4g, a liquid crystal layer 4h, a transparent electrode 4d and a glass substrate 4b disposed in this order, wherein an alternating voltage (AC) of 10 V is applied across the transparent electrodes 4c, 4d from an alternative bias power source 4i.
Accordingly, when a feeble writing light WL of an information image irradiates the photoconductive layer 4e through the glass substrate 4a of the special light modulator 4, an impedance distribution of the photo-conductive layer 4e changes corresponding to the intensity distribution of the writing light WL. Thus, the voltage applied across the transparent electrodes 4c, 4d is controlled accordingly with the change of the impedance distribution of the photoconductive layer 4e. On the other hand, when a light KL from the reading light source 5 irradiates on the spatial light modulator 4 through the glass substrate 4b, the light KL passing through the liquid crystal layer 4h is reflected by the dielectric mirror 4g and proceeds in the reverse direction as a reading light RL.
During the light KL is passing through the liquid crystal layer 4h and is reflected by the dielectric mirror 4g as a reading light RL, the reading light RL undergoes an optical modulation in accordance with the polarization state of the liquid crystal layer 4h which is changed corresponding to the change of the intensity distribution of the writing light WL.
Accordingly, the spatial light modulator 1 has a feature that the writing light and reading light can be processed independently each other without any interaction to each other due to the light shielding layer 4f for optically shielding the light KL from invading the photoconductive layer 4e, though the reading light RL is modulated in accordance with the intensity of the light distribution of the writing light WL.
At that time, a feeble light can be used for the writing light WL because of employing an amorphous silicon material having a high sensitivity as the photoconductive layer 4e. Thus, when the CRT is employed, the amount of the electron beam current of the CRT can be reduced, and when the liquid crystal panel is employed, the driving current of the liquid crystal panel can be made at the minimum. These enable to write a high resolution image thereon.
Referring back to FIG. 1, upon operating the video projector 1 equipped with the above spatial light modulator 4, the image displayed on the CRT 2 is focused onto the photoconductive layer 4e of the spatial light modulator 4 by the writing lens 3 through the glass substrate 4a.
On the other hand, when the light KL from the reading light source 5 impinges on the polarization beam splitter 6, an S-polarized component light of the light KL is selectively reflected at a right angle and is incident on the spatial light modulator 4 through the glass substrate 4b and is reflected and outputted as the reading light RL being modulated by the liquid crystal layer 4h in accordance with the modulation state caused by the writing light WL.
When the reading light RL outputted from the spatial light modulator 4 impinges on the polarization beam splitter 6, only a, P-polarized component of the reading light RL passes through the beam splitter 6 and is outputted as an image light corresponding to the modulation state of the liquid crystal layer 4h, and is projected onto the screen 8 as a magnified image by the projecting lens 7.
Incidentally, the polarized beam splitter 6 is formed in a cube by using glass materials, and a dielectric multiple layer 6a is formed along a diagonal line of the cube in the glass materials.
Next, a description is given of a color video projector 10 in the prior art equipped with a plurality of CRTs and a plurality of spatial light modulators according to the principle of the above video projector 1 in reference with FIG. 3, wherein like components seen in the video projector 1 are shown by corresponding reference characters, and components exclusive to R, G and B are respectively shown by adding auxiliary corresponding reference characters, R, G and B.
In the color video projector 10 shown in FIG. 3, color images of R, G and B each displayed on the exclusive CRTs 2R, 2G, 2B (here only 2R is shown) for R, G and B colors are respectively focused onto the spatial light modulators 4R, 4G and 4B by the writing lenses 3R, 3G and 3B.
On the other hand, after the light KL emitted from the light source 5 is reflected by the mirror 11, the light KL is divided into three lights in accordance with B, G, R wavelength ranges in this order by dichroic filters 12B, 12G, 12R. The separated lights are incident on polarization beam splitters 6B, 6G, 6R reflected by mirrors 13B, 13G, 13R, respectively. The reading lights of B, G and R modulated by the spatial light modulators 4B, 4G, 4R are outputted through the polarization beam splitter 6B, 6G, 6R, and are projected onto a screen (not shown) as a composite color image by a plurality of projection lenses 7R, 7G, 7B.
As mentioned in the foregoing, it is difficult to obtain an image having both high resolution and high brightness by employing the method where an image displayed on the CRT is directly projected optically onto the screen by being magnified or the method where an image displayed on the liquid crystal panel is directly projected optically on the screen by driving the small type liquid crystal panel as a light valve.
On the other hand, when employing the method as shown in FIG. 3, wherein the reading lights of R, G, B are projected by the plurality of projecting lenses to compose a single composite color image on the screen. Thus, it will be understood that it is inconvenient for users to adjust the reading light of R, G, B to converge for producing a single composite image on the screen by projecting them actually. Furthermore, this may require a person who is skilled in adjusting the reading lights of R, G, B.
Further, it is difficult to employ zoom lenses in the method because the three projection lenses 7R, 7G, 7B are used. When fixed focusing lenses are used as the projection lenses, a length between the screen and the lenses is limited, thus the space or place for projecting the images on the screen is also limited.
Further, there is a problem of a high production cost because the video projector 10 employs three projection lenses 7R, 7G, 7 B.